Problem: In geometry class, the girl to boy ratio is $9$ to $6$. If there are a total of $45$ students, how many boys are there?
Answer: A ratio of $9$ girls to $6$ boys means that a set of $15$ students will have $9$ girls and $6$ boys. A class of $45$ students has $3$ sets of $15$ students. Because we know that there are $6$ boys in each set of $15$ students, the class must have $3$ groups of $6$ boys each. There is a total of $18$ boys in geometry class.